The drama follows the lives of undercover police agents who must live a double life in secrecy. Not knowing when they can return to their normal life, they must also endure the constant fear of being uncovered by those they are assigned to infiltrate. However, this time their lives are put in danger when it is discovered that a mole lives within the police force.

Living in the guilt of sacrificing his informant in a previous operation, Criminal Intelligence Inspector Don Lee is wary when his superior orders him to send another informant to spy on the criminal operations of Barbarian, a vicious gangster plotting a jewelry heist. He seeks ex-convict Ghost, who agrees to work for Don despite vowing to go straight since he's desperately in need of money to repay a debt and save his sister. Ghost's driving skills help him infiltrate Barbarian's gang and earn his trust, but by feeding information to Don and hooking up with Barbarian's girlfriend Dee, Ghost is pushing himself into a tight and deadly corner...

Chan Wing Yan, a young police officer, has been sent undercover as a mole in the local mafia. Lau Kin Ming, a young mafia member, infiltrates the police force. Years later, their older counterparts, Chen Wing Yan and Inspector Lau Kin Ming, respectively, race against time to expose the mole within their midst.